Energy Conservation between Natural Ventilated and … Term--Thermal comfort; Clothing value; Energy ... Thermal comfort measuring ... IEQ survey instruments Measuring instrument IEQ

International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 14 No: 06 5

144906-7676-IJCEE-IJENS © December 2014 IJENS

I J E N S

Energy Conservation between Natural Ventilated and

Air-conditioned Classroom in Taiwan Meng-Chieh Lee

1*, Ling-Tim Wong

2, Kwok-Wai Mui

2, Wai-Hou Lam

1, Chia-Feng Chang

1

1 Department of Interior Design, National Taichung University of Science and Technology

2 Department of Building Service, The Hong Kong Polytechnic University

* Corresponding e-mail: [email protected]

The energy consumption of natural ventilated classrooms (NVs)

is less than air-conditioned classrooms (ACs) in hot and humid

Taiwan. Energy conservation is related with the thermal comfort

of occupants in these classrooms. This study proposed an

electronic questionnaire and instruments to facilitate the

continuous monitoring of the physical parameters to investigate

the satisfied level of thermal comfort of students in classrooms.

The investigated results show the average clothing value (clo)

around 0.7. During the hot days (clo<0.7), students’ acceptable

range of temperature range in NVs was between 26.2℃–28.2℃

(neutral temperature=27.3℃, neutral humidity=59.9%) and

24.2℃–27.7℃ (neutral temperature =26.3℃, neutral

humidity=57.2%) in ACs. During the cold days (clo≧0.7),

students’ acceptable range of temperature in NVs was between

20.9℃–25.1℃ (neutral temperature = 23.0℃, neutral

humidity=55.3%) and 22.0℃–25.4℃ (neutral

temperature=23.7℃, neutral humidity=55.6%) in ACs. The

maximum acceptable thermal temperature of students in NVs is

1℃ higher than that in ACs and 2℃ higher than the ASHRAE

Standard 55. The result implies that turn on the ceiling fans in

NVs can be saved 5.0kWh (95%) energy between NVs and ACs

under the occupants’ acceptable temperature range to achieve

the balance between the energy saving and comfortable

environment.

Index Term-- Thermal comfort; Clothing value; Energy

conservation; Natural ventilated classrooms; Air-conditioned

classrooms

NOMENCLATURE

A floor area (m2)

C constant (-)

E energy consumption (kW/h)

EE efficiency of energy saving (%)

EER energy efficiency ratio of split type air-conditioner

(-)

K constant of air-conditioner energy consumption

(kW/h)

N number of samples

T mean temperature(℃)

opT~

operative temperature in each data(℃)

V wind velocity (m/s)

Greek

φ percentile of two occupant groups Gi at the

perceived operative temperature

σ standard deviation

μ mean value

θ occupant‟s sensation (<0 represents “feeling cold”,

>0 represents “feeling hot”)

Subscript

a air temperature

c air-conditioners

n ceiling fans in natural ventilation room

op operative temperature

r radiant temperature

t total



I J E N S

1. INTRODUCTION

In order to resist the hot and humid climate, air-conditioner

becomes an essential equipment to maintain a fine learning

environment in Taiwan college classrooms. However,

air-conditioning consumes the most energy in classrooms (the

second is the lighting equipment and fans consume the least)

[1]. For energy saving, schools in Taiwan follow the

recommendation of air-conditioner setting at 26℃-28℃. As

the temperature setting directly influences the thermal comfort

of occupants, the balance between the energy saving and the

comfortable thermal environment is analyzed in this study.

Many scholars around the world have studied thermal comfort

assessment methods and standards. Fanger proposed a

PMV-PPD model to assess the satisfaction of thermal

environment, as shown in Fig. 1[2]

. ASHRAE Standard 55

proposed the summer thermal comfort standard of 23℃-26℃,

while the clothing value is 0.5 and relative humidity is 60% [3]

.

However, people in Taiwan used to the hot and humid climate.

As metabolic rate is influenced by the different living habit,

ASHRAE Standard 55 may not suit a subtropical region. Chen

[4] found that students in Taiwan can accept a higher

temperature environment in a classroom and only a few

students requested for a lower temperature. Kwok [5]

found

that 75% acceptable temperatures of students in Hawaii do not

conform the ASHRAE Standard 55. These researches prove

that thermal comfort is also deeply influenced by the different

environment and living habit [6]

.

Thermal comfort is basically influenced by the four physical

parameters (air temperature, radiant temperature, relative

humidity and air speed) and two personal parameters [7]

(metabolic rate and clothing values). Occupants‟ thermal

comfort is an important parameter to the temperature setting

of air-conditioners [8]

. Students in classroom are in sedentary

activity and the metabolic rate of each student is almost the

same. In condition of irreversibility of the indoor physical

environment, students usually adjust their clothing values or

switch on the air-conditioners or fans in order to satisfy their

thermal comfort. Ceiling fan is always installed in natural

ventilation classroom because of the low energy consumption.

This study discusses the suitable range of thermal comfort in

natural ventilated classrooms (NVs) and air-conditioned

classrooms (ACs) in Taiwan via the different clothing value of

occupants and proposes the optimum energy saving efficiency.

Fig. 1. PMV /PPD relationship



I J E N S

2. METHODOLOGY

Thermal comfort measuring instrument

Field surveys were always restricted by the scale of

instruments. This study adopts the IEQ survey instruments,

which consist of a portable IEQ system and a touch tablet PC

for asking the IEQ sensation vote, as shown in Fig. 2 and

Table I. These instruments can automatically measure and

record physical parameters in classrooms. The dimension is

small and handy that can be operated in different position. The

IEQ survey instruments are placed on the desk at a height of

100 – 110cm [8]

, as shown in Fig. 3. The IEQ sensation vote

directly inputted by occupants could reduce the artificial

incorrect data typing into the computer analysis software, such

as MS-Excel. According to the former proposed advantages,

the IEQ survey instruments and the tablet PC with electric

questionnaire can collect the objective physical parameters

and subjective thermal sensations questionnaire from the

occupants effectively [9][10][11][12][13]

.

Fig. 2. IEQ survey instruments

Fig. 3. The measuring position of the field survey [8]



I J E N S

Table I

IEQ survey instruments

Measuring instrument IEQ factors Physical parameters Subjective sensation vote

Thermal comfort

Air temperature Clothing value

Radiant temperature Thermal sensation vote

Relative humidity Thermal satisfaction

Indoor air quality Carbon dioxide IAQ sensation vote

IAQ satisfaction

Visual comfort Illumination level Visual sensation vote

Visual satisfaction

Aural comfort Background noise level Aural sensation vote

Aural satisfaction

Questionnaire design

The electronic questionnaire refers to the subjective sensation

of occupants and is clearly separated into two zones, basic

information and subjective sensation votes. Basic information

of occupants, such as gender and age, other background

environment condition with operation of air-conditioning and

opening windows, is the important parameters for analysis.

This study focuses on the thermal comfort via thermal

sensation, humidity sensation, windy comfort and clothing

values. The model of Predicted Mean Vote (PMV) is applied

to assess the subjective environment response and the scale of

thermal comfort is classified into 7 levels, ranging from „Cold‟

(－3) to „Hot‟ (+3) with a “＋” sign representing the hot side

and a „－‟ sign representing the cold side, as shown in Table

II.

Table II

Questionnaire design

Basic information

Gender/ Age/ Air-condition/ Windows/ Clothing value (0.1, 0.3, 0.5, 0.8, 1.0, 1.5, 3.0)

Subjective sensation votes

PMV level -3 -2 -1 0 +1 +2 +3 Acceptance

Thermal comfort Cold Cool Slightly

cool Neutral

Slightly

warm Warm Hot

Accept

or

Not accept

Humidity comfort Very dry Dry Slightly

dry Neutral

Slightly

wet Wet Very wet

Windy comfort Calm Breeze Slightly

breeze Neutral

Slightly

wind Wind Stiff wind

Overall

assessment Very bad Bad

Slightly

bad Neutral

Slightly

good Good Very good

3. FIELD SURVEYS

Field surveys were conducted in Taiwan collage classrooms

from 2010 winter to 2012 winter. This study adopted random

sampling. After deduction of invalid samples (e.g. incorrect or

incomplete measurements), 683 valid student samples were

obtained. Classrooms were classified into natural ventilated

The scope of this study



I J E N S

classrooms (NVs) and air-conditioned classrooms (ACs). 263

students were in NVs and 420 students were in ACs, as shown

in Table III. The area of most classrooms is 80m2 (10m x 8m)

with a capacity of about 50 occupants, some classes studies

were half occupied since they were elective subjects .

Table III Data of field surveys

Natural ventilated

Classroom N01 N02 N03 N04 N05 N06 N07 N08 N09

Clothing value

(clo) 0.99 0.96 0.85 0.85 0.66 0.65 0.60 0.57 0.47

Operative

Temperature (℃) 19.6 19.4 21.9 21.9 27.4 27.1 27.7 28.0 29.1

Relative

Humidity (%) 55.2 55.1 64.9 50.8 64.2 62.2 57.0 60.2 72.9

Air Speed (m/s) 0.3 0.27 0.19 0.15 0.21 0.3 0.22 0.3 0.32

Area (m2) 80 80 80 80 68 77 104 100 80

No. of students 37 37 42 24 16 44 22 21 20

Density (m2/p) 2.2 2.2 2.0 3.3 4.3 1.8 4.7 4.8 4.0

According to the field survey, 70% classrooms were equipped

with 3 air-conditioners and 6 ceiling fans, as shown in Fig. 4.

When the classroom is air-conditioned, ceiling fans are always

turned off. When the classroom is natural ventilated, ceiling

fans are usually turned on for comfort during the hot days.

Students in classrooms were in sedentary activity and the

metabolic rates of each student were referred by ASHRAE

Standard 55 in 1.0met - 1.2met approximately [8]

. This study

found that students usually adjust their clothing values even at

cold days or hot days when the thermal environment was

unable to be changed. Hence, this study averages the clothing

values of each student and determines the relationship

between the clothing value and thermal comfort in classrooms.

Subjective sensation questionnaire and instruction of the

investigation was operated after 15mins of the lesson for

stable condition of occupants. The IEQ survey instruments

were put on the desk of each student, as shown in Fig. 5. In

the meantime, students answered the electronic questionnaire

according to their instant sensation around their position of the

classroom environment. The electronic questionnaire and IEQ

Air-conditioned

Classroom A01 A02 A03 A04 A05 A06 A07 A08 A09 A10 A11 A12 A13 A14 A15 A16

Clothing value (clo) 0.99 0.86 0.75 0.65 0.63 0.58 0.58 0.52 0.51 0.48 0.47 0.47 0.45 0.45 0.44 0.42

Operative

Temperature (℃) 22.0 20.6 24.5 25.5 25.2 25.9 26.7 25.0 24.4 25.8 27.9 26.2 26.8 26.1 27.4 26.1

Relative

Humidity (%) 50.4 55.3 62.1 54.1 58.0 49.0 53.7 57.4 62.8 56.6 62.5 60.4 55.4 61.1 59.1 60.0

Air Speed (m/s) 0.21 0.10 0.12 0.18 0.24 0.24 0.13 0.18 0.19 0.13 0.19 0.18 0.13 0.12 0.33 0.18

Area (m2) 80 80 125 111 158 80 72 80 80 80 80 68 80 80 80 80

No. of students 24 28 34 30 26 16 46 33 21 27 23 23 13 20 28 28

Density (m2/p) 3.3 2.8 3.7 3.7 6.1 5.0 1.6 2.4 3.8 2.9 3.4 3.0 6.2 4.0 2.8 2.9



I J E N S

survey instruments were passed to another student after the

first student finished the questionnaire, and so on. The

operation time of each questionnaire was 2 minutes

approximately.

Fig. 4. The condition of classroom in college

Fig. 5. The field survey and the IEQ instruments



I J E N S

4. RESULT

According to the result from the questionnaires, the average

clothing value was 0.7clo (Number of samples = 683, standard

deviation = 0.27) and this value can be used to determine the

boundary of the thermal environment conditions. When the

clothing value is lower than 0.7 (clo<0.7), the thermal

environment is defined as a hot day. On the contrary, the

thermal environment is defined as a cold day when the

clothing value is higher than 0.7 (clo≧0.7). Fig.6 describes

the relationship between clothing values and operative

temperature where students changed their clothing values

according to the operative temperature.

Fig. 6. The analysis of clothing value

Fig. 7a, b shows the percentages of students‟ thermal

environment dissatisfaction in natural ventilated classrooms

(NVs) and air-conditioned classrooms (ACs) through the

thermal sensation vote (TSV) analysis. When TSV = 0, almost

all students can accept the neutral thermal environment (99%

in NVs and 97% in ACs). In NVs, no student voted “Very hot”

and all students who voted “Very cold” did not accept the

thermal environment. In ACs, only 50% students who voted

“Very cold” did not accept the thermal environment and when

TSV = +3, all students voted “Unacceptable”. The result

shows that students in ACs expect for a cooler environment,

comparing with NVs.

Fig. 7a. Students‟ thermal acceptance in NVs Fig. 7b. Students‟ thermal acceptance in ACs



I J E N S

Fig. 8a, b shows the percentages of students‟ humidity

environment dissatisfaction in natural ventilated classrooms

(NVs) and air-conditioned classrooms (ACs) through the

humidity sensation vote (HSV) analysis. When HSV = 0,

almost all students can accept the neutral humidity

environment (97% in NVs and 96% in ACs). In NVs, no

student voted “Very wet”, some students also liked “Slight

dry” and “Dry” environments (88% and 80%) when HSV = 0,

and there is no one accepted “Very dry” environment. In ACs,

33% students voted “Very dry” and only 25% students voted

“Very wet”, most students liked the “Neutral”, “Slight dry”,

and “Slight wet” environments (96%, 68%, and 65%) when

HSV = 0, but dryer environments are more acceptable. The

result shows that students in NVs expect for a dryer

environment, comparing with ACs.

Fig. 8a. Students‟ humidity acceptance in NVs Fig. 8b. Students‟ humidity acceptance in ACs

Based on the accepted results in thermal and humidity

environments, the thermal and humidity perception of students

may be affected by customary sensation for higher expectation

in Air-conditioned classrooms.

In order to find out the neutral temperature and the acceptable

range for students in Taiwan, this study applied the equation of

the preferred temperature for the surveyed occupants and

interpretations of the thermal sensation votes, proposed by

Mui and Wong [7]

, as shown in Equation (2). The respondents

were presented into two groups, group G1 voted for preference

„warmer‟ and group G2 voted for preference „cooler‟. The

percentile φ of the two occupant groups Gi at the perceived

operative temperature approximated by normal distributions

with mean and standard deviation. The operative temperature

equation proposed by ASHRAE Standard 55, as shown in

Equation (3). The air velocity is also considered by thermal

comfort evaluation, the average air velocity is 0.25 m/s in NVs,

and 0.18 m/s in ACs.

opT

opiopiopop dTTii ,, ,~

322 ;

0;2

0;1

i

(2)

ropaopop TCTCT )1(

(3)



I J E N S

16.0;7.0

6.02.0;6.0

2.0;5.0

V

V

V

Cop

Hot days

In hot days (clo<0.7), 117 students in natural ventilated

classrooms (NVs) and 286 students in air-conditioned

classrooms (ACs) were surveyed. The result implies that

students prefer switching on air-conditioner during the hot

days in subtropical Taiwan. According to the subjective

questionnaire result in NVs, the average neutral temperature

was 27.3℃ (standard deviation = 0.5℃, neutral humidity =

59.9%) . Air-conditioners are utilized to maintain a

comfortable thermal environment. In ACs, 159 students (55%)

felt comfort (PMV=0). 82 students (29%) felt cold (PMV<0)

and 45 students (16%) felt hot (PMV>0). The neutral

temperature was 26.3℃ (standard deviation = 0.7℃, neutral

humidity = 57.2%). The maximum acceptable temperature

with standard deviation in NVs is 27.8℃, and in ACs is 27℃,

as shown in Table 4. The result implies that students in NVs

can tolerate a higher temperature during the hot days.

Moreover, they can accept a wider range of temperature and

also accept a higher range of humidity, as shown in Fig. 9a, b,

and Fig. 10a, b.

Table IV

The acceptable range of temperature and relative humidity during the hot days

Hot days (clo <0.7)

Ventilation type No. of

Samples Operative temperature (℃) Relative humidity (%)

Natural ventilated 117 Neutral temperature = 27.3℃

Standard deviation = 0.5℃

Neutral humidity =59.9%

Standard deviation = 4.3%

Air-conditioned 286 Neutral temperature = 26.3℃


Neutral humidity = 57.2%


Fig. 9a. b. Thermal sensation votes in hot days



I J E N S

Natural ventilated (hot days) Air-conditioned (hot days)

Fig. 10a. b. Humidity sensation votes in hot days

Cold days

In cold days (clo≧0.7), 146 students in natural ventilated

classrooms (NVs) and 134 students in air-conditioned

classrooms (ACs) were surveyed. The neutral temperature of

students in NVs was 23℃(standard deviation = 2.1℃, neutral

humidity = 55.3%). This result is similar to the ASHARE

Standard 55 about the comfort zone in cold days. In Taiwan,

the average outdoor temperature in cold days is relatively high,

and hence, heating equipments are not suggested to be used in

order to save energy. In ACs, all students wore sweaters or

other outer garment to protect from the cold temperature. The

average neutral temperature was 23.7℃, (standard deviation =

2.4℃, neutral humidity = 56.1%) as shown in Table V. The

neutral temperature in ACs is higher than it in NVs, because of

switching on the air-conditioner to provide the air-flow

without open windows for keeping warmer classroom with

ventilation, as shown in Fig. 11a, b. In Taiwan, the relative

humidity in cold days (dry season) is not significant difference

between NVs and ACs, because outdoor relative humidity is

under the comfort zone (40%-60%), as shown in Fig. 12a, b.

Since the diurnal temperature in cold days is higher, some

classrooms always turn on the air-conditioner even in cold

days. However, it is not recommended for turning

air-conditioner in cold days. Adopted natural ventilation can

be saved more energy from air-conditioner for heating,

cooling and dehumidifying.

Table V

The acceptable range of temperature and relative humidity during the hot days

Cold days (clo≧0.7)

Ventilation type No. of

Samples Operative temperature (℃) Relative humidity (%)

Natural ventilated 146 Neutral temperature = 23℃




Air-conditioned 134 Neutral temperature = 23.7℃






I J E N S

Natural ventilated (cold days) Air-conditioned (cold days)

Fig. 11a. b. Thermal sensation votes in cold days

Natural ventilated (cold days) Air-conditioned (cold days)

Fig. 12a. b. Humidity sensation votes in cold days

5. DISSCUSSION

Thermal comfort zone

Taiwan belongs to the subtropical climate zone and is

surrounded by the ocean, causing a high temperature and high

humidity climate. Especially summer (hot days) is usually

raining in the wet season and winter (cold days) is usually in

dry season. As Taiwan students can accept a higher

temperature and humidity for fitting local climate than that in

the ASHRAE Standard 55.

Fig. 13a, b shows the thermal comfort zone of NVs and ACs

in subtropical Taiwan. During the hot days, no occupant in

NVs fell on the comfort zone of ASHRAE Standard 55 and

they can tolerate the temperature range at 26.2℃ - 28.2℃

(relative humidity = 54.1% - 65.2%). In ACs, 60% occupants



I J E N S

fell on the comfort zone of ASHRAE standard 55 and the

acceptable range of temperature was between 24.2℃ - 27.7℃

(relative humidity = 43.9% - 69.3%). The students usually stay

in the hot and humidity climate (the outdoor relative humidity

in Taiwan is usually around 70% in the hot days (wet season)

and still over 40% in the cold days (dry season)), they can

accept higher temperature and humidity in the hot day,

especially in NVs without much thermal comfort expectation.

During the cold days, 68% occupants in NVs fell on the

comfort zone of ASHRAE Standard 55 and the acceptable

range of temperature was between 20.9℃ - 25.1℃ (relative

humidity = 47.5% - 64.9%). 54% occupants in ACs fell on the

comfort zone of ASHRAE Standard 55 and the neutral

temperature setting is 23.7℃ under the acceptable range of

temperature between 22.0℃ to 25.4℃, and the acceptable

range of relative humidity between 43.9% to 67.1%.

Compared with the ASHRAE Standard 55 (comfort

temperature = 23℃ - 26℃, relative humidity = 40% - 60%),

students in NVs have a higher acceptable range of temperature

and relative humidity, because the outdoor relative humidity is

still higher than 40% in dry season.

Fig. 13a. Thermal comfort zone in hot days Fig. 13b. Thermal comfort zone in cold days

Energy efficiency

Students in NVs can accept the comfort temperature 1℃

(mean temperature) higher than it in ACs and accept around 2

℃ higher than it with same relative humidity in ASHRAE

Standard 55 during the hot day. According to the previous

operated experiences in Taiwan, energy consumption of an

air-conditioner and a ceiling fan is approximately 0.18kWh/m2

and 0.04kWh respectively, and cooling 1℃ more to consume

6% energy[14]

. Based on the field survey result, many

classrooms were equipped with 3 air-conditioners and 6

ceiling fans. By the calculation of Equation 4, the total energy

consumption of air-conditioning is 5.26kWh when the floor

area is 80m2 and the EER of a split type air-conditioner is 2.73.

The energy consumption of ceiling fans is 0.24kWh.

According to the American Standard, 3 air-conditioners with

4,032kcal/h or an air-conditioner with 12,096kcal/h is needed

to support the whole classroom cooling. By the calculation of

Equation 5, 5.0kWh (95%) energy consumption can be saved

in natural ventilation classrooms comparing with

air-conditioned classroom in Taiwan, and it can be saved more

than 5.7kWh comparing with ASHRAE standard 55 comfort

zone in the same relative humidity and tolerating comfort

temperature during the hot days.

EERKAE tc (4)



I J E N S

%1001

c

nE E

EE (5)

6. CONCLUSION

This study aims at the thermal comfort of students in

subtropical classrooms in Taiwan. 683 samples were received

through field surveys. This study proposes the difference of

thermal temperature range between the natural ventilated

classrooms (NVs) and air-conditioned classrooms (ACs) via

the clothing values and analyzes the efficiency of energy

saving.

According to the result, 0.7clo is the boundary to distinguish

hot days and cold days since students adjust their clothing

value to resist the thermal environment. As students in Taiwan

used to tolerate the hot and humid climate zone, most students

can accept a high temperature and high humidity environment

in a classroom. In hot days, the neutral temperature and

humidity of students in NVs were 27.3 ℃ and 59.9%

respectively. 71% classrooms switched on the air-conditioner

and students in ACs can only accept a lower and smaller range

of temperature by expectation. NVs can save 5.0kWh (95%)

energy compared with ACs and 5.7kWh compared with

ASHRAE Standard 55. Hence, under the acceptable range of

temperature, ceiling fan is recommended in natural ventilation

classrooms to achieve the balance between the energy saving

and comfortable environment. In addition to the passive

strategy, internal or external shading, heat isolation and proper

ventilation would be a fundamental issue of energy saving.

7. ACKNOWLEDGEMENT

The author great thank National Science Council “NSC

100-2221-E-025-012“, “NSC 98-2221-E-025-010“, “NSC

97-2221-E-011-119“, supporting the financial to investigate

the students‟ thermal comfort in the university classrooms.

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https://nscnt12.nsc.gov.tw/RS09/Modules/Personal/Personal_Researcher_index.aspx



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Energy Conservation between Natural Ventilated and … Term--Thermal comfort; Clothing value; Energy ... Thermal comfort measuring ... IEQ survey instruments Measuring instrument IEQ